In recent times, the use of electrosurgical instruments to perform precise surgery has become widespread, mainly because it offers significant advantages over traditional scalpel surgery. For many surgical procedures, e.g., open heart surgery and laparoscopic procedures, electrosurgery results in significantly less patient trauma. In neurosurgical procedures, because of the very sensitive tissue around the surgical site and the need to immediately limit bleeding, the facility to perform incisions and coagulations with a single electrosurgical instrument is highly advantageous. Similarly, in transurethral surgery, where the goal is to safely remove unwanted or diseased tissue from a prostrate gland, the operation is best done with an electrosurgical instrument because the surgeon has to operate through a relatively narrow bore resectoscope.
It is well known that conventional electrosurgical instruments operate when an electrical path is completed between two electrodes. This requires that both electrodes be in some kind of contact with the patient's body. The electrical current provided to such instruments is sinusoidal, i.e., neither electrode is either positive or negative relative to the other except instantaneously.
Most surgeons prefer to use electrosurgical instruments over traditional scalpels because the former are essentially "bloodless" knives. Such an instrument can dissect tissue while at the same time reducing the amount of blood loss by permitting quick coagulation of the dissected tissue. Cutting occurs when the current density is high enough to explode the tissue cells near the electrode, and the same instrument may have a portion usable to either seal an incised vessel or to accelerate the coagulation process.
As best seen with reference to FIG. 1, when the sinusoidal waveform of the applied current is continuous and at its maximum amplitude, a cutting effect is obtained due to intense, almost explosive heating of cells contacted by the active electrode due to the locally very high current density. To provide coagulation, a damped current waveform, comprising short bursts of current rather than a continuous current, may be provided to the tissue. This causes local cellular dehydration because of the smaller amount of electrical current and power delivered to the tissue. It allows the surgeon to obtain hemostasis, allowing him to destroy tissue masses, or to cause selective desiccation of tissue. For a blended operation the surgeon may apply a current having a waveform as indicated at the right portion of FIG. 1.
As schematically indicated in FIG. 2, providing continuous high amplitude current via a thin electrode can generate a pure cut, and applying a damped waveform can generate a blended cut in which there is incision of tissue as well as coagulation of any blood leakage nearby. By selecting an appropriately shaped active electrode tip, applying a low power flow and holding the tip in physical contact with tissue, the surgeon can cause local heating and desiccation of the tissue without necessarily causing tissue necrosis. Finally, by holding the electrode close to, but not in direct contact with the tissue while applying a high power flow, a surgeon can cause arcing between the electrode and the tissue surface to cause local charring to produce fulguration. This last technique is commonly used in cardiovascular and thoracic surgical procedures.
A monopolar instrument is one in which the electrical current flows from the active electrode, which may be shaped as a blade, a hook, or a straight ended wire, to an electrical "ground". A grounding pad is typically applied to the thigh, back or some other spot on the patient's body where contact may be made with a relatively large surface area. Thus, in monopolar electrosurgical instruments, the surface area of one of the two electrodes is deliberately made significantly larger than that of the other. This is done so that the current density is much greater at the electrode having the smallest surface area when in contact with the patient's tissue. In such applications, the electrode with the smaller surface area is referred to as the "active" electrode. There are, however, serious problems that arise when there are strong current paths established due to deterioration of electrical insulation, capacitive coupling in spite of intact insulation, and accidental arcing between the instrument body and the patient's tissue in regions not visible to the surgeon. Some of these can be solved by the use of electrical shielding between the monopolar surgical instrument and a metal cannula. Monopolar electrosurgery, however, requires that an electric current flow through the patient's tissues away from the surgical site.
The frequency of the electrical voltage used for electrosurgery of any kind is typically much higher than the conventional mains frequency of 60 Hz, often in the range 400,000 Hz to 3,000,000 Hz, a range commonly referred to as "radiofrequency. Radiofrequency is generally considered too high to stimulate muscular tissue and, therefore, is believed to be safe to the patient. However, because the electrical current must flow through a significant portion of the patient's body, the surgeon and staff are at some risk of being shocked because of capacitive coupling with the patient. Also, the high voltage/high frequency generator and the wire leading therefrom to the active electrode, can act as an electrical noise generator and may adversely affect sensitive instrumentation.
In light of the above-described problems with a monopolar instrument, a bipolar instrument is sometimes used. It requires no grounding pad to be applied to the patient, but instead employs two electrodes which mechanically oppose each other like the two halves of a surgical forceps. The instrument is used to squeeze tissue between the two electrodes as a current passes between them to cause coagulation. Thus, a bipolar surgical instrument normally is used only when coagulation is desired and when current flowing through the patient is clearly undesirable. The bipolar instrument is not used to dissect tissue in most cases because it does not permit the surgeon to do so with precision.
Reference to FIG. 3 shows various types of monopolar active electrode tip shapes as well as the general tip end structure of a bipolar instrument. FIG. 4 illustrates the general overall configuration of a bipolar instrument.
FIGS. 5(A)-5(C) schematically indicate the typical distributions of electrical lines of force and equipotential lines normal thereto (as broken lines): for a monopolar instrument having a single electrode contacting the tissue to be operated on; for a bipolar instrument with two electrodes between which current flows through the patient's tissue, and for a coaxial surgical instrument (CSI) as in this invention in which there is a single electrode coaxially surrounded by an outer electrode with the entire operative electrical field highly localized and contained therebetween.
The present invention relates to a particularly advantageous form of a coaxial surgical instrument and has the general form illustrated in FIG. 6 and explained more fully hereinbelow.